Why do trees foam when it rains?


After heavy rainfall, like the severe rainfall and flooding experienced on the Central Coast during 2022, you may have seen something that had you scratching your head….white foam coming from trees in your area.

Weird, right?

Well, don’t be alarmed, this is a process called saponification. Saponification occurs when rainwater mixes with the sap in the tree, creating a soapy foam. This foam is made up of saponins, which are natural surfactants that help the tree absorb water and nutrients from the soil.

Sometimes referred to as ‘tree foam’, this process changes the surface tension of the water, so when it drips down to the base of the tree, turbulence introduces air, and the altered surface tension creates foam.

Tree foam also happens in other areas, like rock (rock foam), and in rivers and oceans, where phosphates combine with water and air to produce foam.

Tree foam occurs in all types of trees and has been noted in all continents where trees grow.

But why are the trees foaming? It’s actually pretty simple. During dry periods, an assortment of particulates, plant chemicals, and air pollutants accumulate on the bark surface of trees. When rain collects on a tree’s canopy, these ingredients mix and concentrate towards the base of the tree. The foam is a result of a crude soap, flowing and bubbling due to turbulence on tree bark furrows. Soap is basically a combination of an alkali metal, such as sodium or potassium, and a mixture of carboxylic acids.

So there you have it, the next time you see a tree foaming, you’ll know why!

It’s always best to be safe when it comes to your trees. This helps prevent catastrophic accidents happening to both buildings and people. Get professional arborist services and keep your property safe for years to come.

Treecorp Solutions has been providing arborist services to the Central Coast, Lake Macquarie, Newcastle and beyond since 1991.

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